MIT-led team cuts noise from landing planes

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Most attempts to reduce the noise associated with landing aircraft are expensive -- modifying aircraft, soundproofing buildings, buying and demolishing homes. But now, an innovative MIT-developed landing procedure is reducing the noise that planes make when landing, while also cutting aircraft operating costs.

Associate Professor John-Paul Clarke and colleagues first tested the new technique at Louisville International Airport in Kentucky in 2002; they found that it cut noise by almost 50 percent and reduced fuel consumption during landing by about 500 pounds.

Since then they have analyzed its application to other airports, including Logan International in Boston, Sacramento Mather in California, and Gatwick in London. Currently the team is developing a certified procedure that the United Parcel Service (UPS) intends to use on its planes in Louisville and Sacramento.

Constraints on aviation

Community concerns about aircraft noise are constraining the growth of aviation. Because of the increasingly active legal opposition to airport expansion by residents in adjacent communities, many runway expansion projects have been delayed or abandoned.

"The net effect is that fewer than five additional runways have been built at the 30 busiest U.S. airports within the last 10 years, resulting in greater delays and congestion," said Clarke, of MIT's Department of Aeronautics and Astronautics.

A number of measures already have been adopted to reduce aircraft noise. These include phasing out noisier aircraft in favor of planes with quieter engine technology; enforcing nighttime curfews on some or all aircraft; and insulating (or purchasing and demolishing) homes that are severely affected by aircraft noise.

While these measures have reduced the impact of aircraft noise, they have not lessened the opposition to airport expansion. Given the relatively wide implementation of the measures, and the potential capacity crisis in the national and international airspace system, there is a critical need for new solutions.

Clarke and colleagues from Boeing Commercial Airplane Group, Boeing Air Traffic Management, NASA Ames Research Center and NASA Langley Research Center believe that one such solution is to change the way aircraft are operated when they're close to airports. To validate their ideas, they designed and flight-tested a continuous descent approach (CDA) procedure for the specific airport and airspace constraints at Louisville International Airport.

Noise is an ongoing problem at Louisville mainly because it's the primary hub for UPS, which lands more than 90 large planes each night. Most of these landings occur between midnight and 2 a.m., when other background noise is low and residents in surrounding communities are trying to get to sleep or have just fallen asleep -- the period when it's easiest for someone to be awoken by noise, according to experts.

In a standard approach, the plane is brought down in stages -- descending and leveling off several times before landing -- with the final level-flight segment only 3,000 feet above the airport. Each time an aircraft descends from an intermediate altitude and levels off, thrust must be applied to maintain level flight. "And increasing thrust increases noise," Clarke said. The resulting noise impact on the ground is even greater in communities such as those in southern Indiana, where the nearby residential elevation is more than 850 feet higher in relation to the local runways than the residential elevation in Louisville, Ky.

The new procedure addresses both the thrust and elevation issues by keeping planes higher for a longer period and then bringing them down in a continuous descent. The aircraft are both quieter (because they are operating at lower thrust levels) and higher as they pass over the affected communities.

Louisville tests

For eight nights, two UPS Boeing 767 airplanes on their way to Louisville from West Coast cities were selected (once all aircraft were airborne) to participate in the flight test, based on their scheduled arrival time in Louisville. Both aircraft needed to be close enough in the sequence that the weather conditions would be the same during their approaches.

One 767 was instructed to perform the standard approach. "The other 767 was instructed to perform our CDA continuous-descent procedure: descending on a two-degree flight path angle and then a three-degree instrument landing system glide slope on the final approach course," Clarke said.

The performances of the aircraft, pilots and flight management system were measured using two separate systems -- the aircraft tracking system installed at Louisville as part of an FAA technology demonstration project, and the onboard flight data recording system. Noise was measured using 14 microphones at seven locations in communities in southern Indiana.

"The results show quite convincingly that the CDA procedure reduces noise between three and six decibels at the measurement locations -- in line with our predictions -- and that there are significant noise benefits for residents living approximately 10 to 30 miles off the end of airport runways when this approach is widely employed," Clarke said.

For reference, a three-decibel difference is noticeable to the average person, while a reduction of 10 decibels is perceived by the human ear as a 50 percent reduction in noise.

Another advantage of the CDA procedure is that it is more fuel-efficient. Because the planes are spending more time at higher altitudes in less dense air and less time in fuel-wasting slow flight configuration, aircraft performing the CDA used approximately 500 pounds less fuel than those using the standard approach procedure.

In addition to testing the procedure at other airports, Clarke's team has been designing cockpit displays and tools for air traffic controllers to enable widespread introduction of the CDA approach.

The work is supported by the FAA and NASA with matching contributions from Boeing and UPS.